Method and apparatus for producing articles from powdered materials



Feb. 3, 1948. H. HQLESTER 2,435,227

METHOD AND APPARATUS FOR PRODUCING ARTICLES FROM PQWDERED MATERIALSFiled Aug. 25, 1942 5-I\ II T (GRAY/TY) l FT 5'- 5- I 6" l 3 W W I IHorace H-Lester w, zgmwdwmmdw W Patented Feb. 3, 1948 METHOD ANDAPPARATUS FOR PRODUCING ARTICLES FROM POWDERED MATERIALS Horace H.Lester, Cambridge, Mass., assignor to the United States of America, asrepresented by the Secretary of War Application August 25, 1942, SerialNo. 456,027

9 Claims. (CI. -22) (Granted under the actor March 3, 1883, as amendedApril 30, 1928; 370 O. G. 757) The invention described herein may bemanui'actured and used by or for the Government for governmentalpurposes, without the payment to me of any royalty thereon.

The present invention relates to owder metallurgy in which variousarticles are formed from powdered metals, their alloys or compounds, andparticularly to the powder metallurgy of articles wherein a dense andcompact structure is the desideratum.

In the formation of articles from powdered metals, the usual processcomprises compacting the mass of powder to a desired shape and size,

usua ly by the application of pressure to the powder whichis containedin a rigid mold; the removal from the mold of the compacted mass, whichafter compacting has sumcient rigidity to retain its shape; and thefurther consolidation of the material by heat, which causes theindividual particles to "sinter" or weld together. The finalconsolidation is usually accompanied by a diminution in volume of thecompacted powder mass.

It is seen that an essential preliminary stage of manufacture is theinitial compacting of the loose powder in the mold. This operationreduces the volume of the loose powder by varying amounts, dependingupon the nature of the powder. In some cases this initial compressionreduces the volume of the loose powder to one-third of its initialvolume.

The initial compacting has heretofore been carried out principally bythe application of pressure to the powder contained in the mold that isat room temperature, in which case the operation is known as coldpressing" or with the mold and powder at an elevated temperature whichmay be the sintering temperature, in which case the operation is knownas hot pressing. Compacting of the loose powder has been accomplished bythe suspension of the powder in a plastic binding material, and theextrusion of the plastic mass through a die by high pressure. The earlytungsten lamp filaments were made in this way.

While some articles of powder metallurgy manufacture can be successfullycompacted by pressure applied to loose powder, as described above,others cannot-for instance, long, slender articles, such as drill rodsor similar dbjects where the length, along which the compressive forcemust be applied, is substantially greater than the transverse dimension.In such cases the movement of the particles in compacting under theapplied pressure develop frictional resistance between each other andbetween the powder parti 2 cles and the walls of the mold. In addition,arches or "bridges are formed which-interfere with compacting and leavesmall voids and layers of high density so that with some articles, thereresults in regions remote from the surface, where the compressive forceis applied, non-uniformities in density that are reflected in the finalsintering operations, and that tend to make the finished articlesunsuitable for use. Also, where the individual powder grains are veryhard, as for instance, particles of. tungsten, titanium, and boroncarbides, they are not crushed or plastically deformed by anycompressive forces that can be applied practically to the powder in themold, so that there necessarily remain interstitial voids between theindividual grains, even though compacted with the highest pressurespracticable to apply. This limitation in the application of pressure forthe compacting of metal powders has been well recognized, and is shownby the A. S. M. Metals Handbook, 1939, (published by the AmericanSociety for Metals) on page 105. Various methods have been employed toovercomethe defeet, such as the incorporation in the powder oflubricants such as parafiin; by the introduction of small increments ofpowder and pressing serially; and by pressing transversely in the caseof slender articles, but such steps are no more than makeshifts insecuring the desired results. For instance, where lubricants, such asparaflin, are employed, the lubricants are decomposed in the sinteringoperation, giving rise to gaseous products or residues which may remainin the finished article as small gas pockets or as impurities.

In addition to the above, objections, the "cold pressing method ofcompacting is necessarily an expensive operation, involving elaboratepresses, and necessarily slows production, and adds materially to thecosts of manufacturing.

It has been found that these objections may be-substantially overcome byfollowing the present invention.

The powdered materials, whether the same be of a pure metal, alloy,compound, or mixtures thereof, with or without a binder material, areplaced in a mold cavity of the desired shape of the finished article.The mold may be of a heatresistant material, such as graphite oraluminum oxide, or in some cases a metal. A plug of smaller size thanthe mold opening, and made of some material such as graphite or metal,and having suflicient mass when the mold is vibrated to produce atamping action, is placed in the mold openmg, The mold-and its contentsare then vibrated either mechanically or electrically by means of asolenoid coil, which acts directly on the mold and contents, or by meansof a steel core inserted in the solenoid, and which acts on the moldthrough the intermediary of a rubber or similar buffer. The object ofthe vibration is to bring about a condition of maximum powder densityand an optimum uniformity of distribution in the particles of thepowder.

By the term vibration I have in mind the short, irregular linear motionsimparted to powder particles through impulses produced either directlyor indirectly by electrical or mechanical means and from collisionsbetween particles. It is recognized that, in addition to the vibratorymotions, as thus defined, the individual particles are subjected todirective forces, either from gravity, centrifugal action, or the actionof the plug as described below which tend to cause a general shift inthe direction of the directive forces.

In the ractice of this invention it is recognized that the vibratorymotions of the individual particles may be derived from variousfrequencies and amplitudes of a vibration of the solenoid or of themechanical vibrator. A sixty-cycle current passed through an ordinarysolenoid in which a steel core was inserted with provision for reulating the amperage of the current has been found to produce suitableresu ts. In use, the magnitude of the oscillat ons of the solenoid havebeen var ed by varyin the amperage thru the coil to produce the optimumpacking as measured by the sinking of the plug into the mold.

If it is assumed that all particles are rounded in contour and of subtantially the same size and shape. a condition of max mum density of theloose powder would occur when every particle has come into as closecontact as is possible, without sinter ng. with its neighbors, and wherethere are no brid es or regions of non-uniform density and whereeachparticle. acting under the influence of gravity has approached asclosely as possible to the center of gravity of the earth, consistentwith its location in the mass of particles and with the supportingforces of the particles beneath it. Such ideal packing could not becompletely realized by any practical method of compactin but asubstantial approach to it could be made if the particles were vibratedand allowed to approach equilibrium positions while in a state ofproperly controlled vibratory motion.

This substantially ideal packing would still leave interstitial voidsbetween the particles which are in contact. These voids would be largerfor larger particles, and smaller for smaller sized particles. If we nowassume a mixture of particle sizes, and if the amounts of the differentparticle sizes were in a correct proportion, there would result, underthe influence of a properly controlled vibration, a condition in whichthe largest voids between the largest particles were fllled bysmaller-sized particles and the voids among the smaller-sized particlesare filled by still smaller-sized particles, and so on, until thesmallest-sized particles have been used in filling the successivelysmaller-sized voids.

Such a condition is ideal and could not be easily realized in practice.However, in any powder mixture containing varieties of particle size,the vibratory motion applied to this mixture, as herein described, andutilizing the mechanical assistance of a plug, and with the assistanceof an additional directive force, such as gravity or centrifugal force,will produce a substantial 3?- proach to the optimum density conditions,and to an optimum description of particle size, which includes anoptimum density distribution.

In addition to the vibration produced either mechanically orelectrically, it has been found highly advantageous to subject theparticles to centrifugal force in a centrifuge, so that each particle inthe mass is subjected to a force which contributes to the compacting ofthe mass. This centrifugal force is superior to an applied pressure inthat each particle is acted upon individually by the centrifugal force,and for this reason the tendency to form bridges in the material islessened. This centrifugal force may be considered as a directive forceinfluencing the linear motions of the particles, which are in states ofvibration; the vibratory motion acting to permit the freer movement ofthe particles in the direction of the centrifugal force.

In addition to the vibration and the directive force which might becentrifugal or gravitational, the action of a plug, inserted asdescribed above. is found to be desirable for the satisfactorycompacting of the material. I

The plug is smaller than the opening in the end of the mold, yet it fitssnugly enough to prevent substantial loss of powder. It is vibratedalong with the mold and powder and usually has a different frequency ofvibration-that is to say, if the mold were vibrated by placing it aboveand in contact with the steel core of the solenoid, gravity. instead ofcentrifugal force, would be the directive force in this case. The plugwould be moved upward and against the force of gravity by an impulsereceived from the vibrating solenoid. The time of the single up and downexcursion would be determined by the acceleration imparted to the bodyby gravity, and would be governed by the laws of falling bodies, exceptin so far as the motion of the plug was influenced by friction with theside of the mold, and influenced by the impinging powder particles fromthe surface of the vibrating powder mass. Since the motion of the plugis governed largely by gravitational forces, it is highly improbablethat the up and down motion thus imparted to the plug would occur withthe same frequency that characterizes the motion of the solenoid, whichis governed by the frequency of the alternating current. It is seen thatthe plug may be regarded as a mechanical vibrator or a mechanical hammerwhich makes impacts on the top surface of the powder mass. It isbelieved that this mechanical action of the plug is essential for thecompacting of the upper layers of the powder. It is believed that thecompacting of the extreme upper layer of the powder mass cannot beaccomplished by vibration alone. While the illustration of this actionof the plug has been based upon the vibration of the powder while underthe influence of gravity, the action of the plug would not be differentin any essential feature, if the directive force were that obtained incentrifuging.

When compacting different metal powders it may be found desirable tocontrol the motion of the plug in the mold in order to vary thecompacting of the upper strata of the metal powder in the mold. Themotion of the plug relative to the mold and/or powder, and consequentlythe density of the upper strata of metal powder in the finished articlemay be controlled by varying the mass of the plug or by damping itsperiod of vibration.

In addition to the above described action of the plug, it is pointed outthat desired shapes may be imparted to the surface of the compactedpowder by suitably shaping o engraving the end of the plug that comesin' contact with the powder.

Summing up, the action of the plug in the mold appears to have thefollowing functions:

1. To secure better compacting, particularly of the layer of powderimmediately adjacent to the plug.

2. To secure suitably shaped bases.

3. To prevent the loss of powder. 7

When the powder is impacted by theapplication of pressure, as in thecold pressing method, the plunger, which is more or less completelyimpervious to gases. fits the mold with sufflcient snugness so that theescape of gases from the powder during compacting is substantiallyimpeded. Thus a certain amount of gas is entrapped in the compactedmass, which fact contributes toward a lowered density of the compactedmass. when compacting is accomplished through the practice of thisinvention, gases are not entrapped to so great an extent in thecompacted mass, due to the fact that the vibrating particles offerlittle resistance to the passage of the gas through the volume of thepowder mass, and the loosely fitting plug offers a minimum of resistanceto their escape from the mold.

By the practice of this invention it is possible, as described above, tosecure favorable preliminary packing and uniformity of packing of thecompacted powder mass. In certain types of ar- .ticles manufactured bythe powder metallurgy process, it is sometimes desired that the finishedproduct retain a certain degree of porosity. In

the past, porosity in the finished product has been secured by theincorporation, in the compacted mass otmaterials which are decomposedduring the sintering operation, thereby leaving small voids, the extentof such voids being controllable by the amount of decomposable materialsincorporated in the powder mass. In the practice of the presentinvention, controlled porosity may be obtained, and the porosityachieved is of a smaller order of magnitude than that produced by theolder methods. For example, if small particles, all substantially of thesame size and roughly spherical in shape, of such materials as pure ironor pure nickel or other weldable metals were compacted in the practiceof this invention, there being little or no binder material and thecompacted mass heated in the usual manner for sintering, the particleswould weld together or "sinter at points of contact, leavinginterstitial voids at points where the particles were not in contact.There would then be a state of final porosity in the finished product.there being cavities or interstitial voids between the adjacent grains.

If a similar operation were carried out with diilerent sized metalparticles from those used in the last illustration, diiierent sizedvoids would occur in the finished product, so that by selecting theinitial particle size, the size of individual voids, and thecorresponding extents of porosity produced can be substantiallycontrolled.

It is pointed out that in the cold pressing method, porosity can not becontrolled in the manner explained, where the powders of ductile metals.such as iron or nickel are employed, except where the compactingpressures are below those which would produce plastic deformation of theindividual grains.

It is pointed out also, that in certain cases as for instance in thecase of sintered carbides. where binder materials are employed, and thebe enough liquid material tofill up the voids,

porosity could be obtained. Hence, it is seen that a condition forcontrolled porosity is the presence of insuflicient binder material tofill up interstitial voids or no binder material.

An apparatus for carrying out the present invention is shown in thedrawing and wherein:

' Fig. 1 is a view principally in elevation of a centrifuge apparatuswith the mold holders in position assumed when the apparatus is inopera- 1 tion.

Fig. 2 is a partial top plan view partly in horizontal section of thestructures shown in Figure 1.

Figure 3 is a view principally in elevation of an apparatus forvibrating the mold and contents, with the central part of the solenoidshown in section.

The centrifuge comprises a head I and depending therefrom is a shaft bywhich the head is supported and rotated. The drive for the shaft may beof any particular kind and the speed controlled by suitable means as iswe'll known to those skilled in the art. The head I supports the pivotedtrunnion cups 2 which may be formed of any material desired. Itdesirable the cup may be formed of a non conductor, as such avoidsheating of the material of the cup by induction and also permits of asimple construction for the conduction of electrical energy to thesolenoid. The solenoid 3 is supported in the cup and is slidable thereinfor the ready removal of the same and the insertion of another solenoid.Concentric with the solenoid 3 is a cup of magnetic material 5 and inwhich is inserted the mold 6. The cup 5 is supported on a bufi'er i madeof rubber, cork, wood or other resilient material. The mold cavity ofthe mold 6 may be made of any shape desired and will be the shape thoughnot of the exact dimensions of finished product to be produced. Thepowdered material 8 to be compacted is inserted in the mold and a plug Ewhich is loosely slidable in the mold is placed on top of powderedmaterial. The electric current from an alternating current source ii iscontrolled by switch 9 and supplied thru a variable resistance It andthe leads 2 to the solenoid 3.

In operation the powdered material is inserted in the mold and a plugplaced on top of the powdered material. The powder is compacted by thejoint action of vibration, centrifugal force and the plug. With theapparatus shown in Figures 1 and 2 the compacting of the powder may takeplace with or without the use of centrifugal force in view of the factthat the trunnion cups are pivoted in the centrifuge head. If thecentrifuge head is not rotated, the solenoid and the mold are held in avertical position by gravity and with gravity acting as the directiveforce on the powdered material. If the centrifuge head is rotated themold is held by centrifugal force in a horizontal or substantiallyhorizontal position so that centrifugal force is the directive forceapplied to the powdered material.

The apparatus shown in Figure 3 is substantially a duplicate of thesolenoid and mold structure used in the centrifuge. The solenoid 3'surrounds the cup 5' of magnetic material and the buffer 4'. The moldfl'iis of a shape to form a bullet of the powdered material 8'. Thepowdered material may be tungsten carbide. The electrical energy tovibrate the mold and the plug I is supplied by an alternating currentsource ll and controlled as to the current flowing in the circuit bymeans of the variable resistance l and switch 9'.

The present invention has been described in conjunction with thepreferred embodiments but it is to be observed that modifications andvariations may be resorted to as one skilled in the art can readilyunderstand.

I claim:

1. The method of producing an article from powdered materials whichcomprises placing the powdered material in a mold, vibrating saidmaterial and simultaneously with said vibrating action rotating saidmaterial about an axis normal to the direction in which the material isbeing vibrated to compact the material in the mold and thereaftersintering the compacted material to form an article of a coherent solidmass having a shape corresponding to that of the mold.

2. The method of producing an'article from powdered materials whichcomprises placing the powdered material in a mold, applying pressure tosaid material by centrifugal force while subjecting said material to a.vibrating motion in the direction of said force whereby the material iscompacted in the mold, and thereafter sintering the compacted materialto form. an article of a coherent solid mass having a shapecorresponding to that of the mold. 3. The method of producing an articlefrom powdered materials which comprises placing the powdered material ina mold, subjecting the material to a vibratory force and simultaneouslyto a centrifugal force acting in a direction parallel to the directionof the vibratory force to compact the material in the mold andthereafter subjecting the compacted material to a sintering heat wherebyan article of a coherent solid mass having a shape corresponding to thatof the mold is formed.

4. The method of producing an article from powderedvmaterials whichcomprises placing the powdered material in a mold, vibrating saidmaterial and simultaneously with said vibrating motion rotating saidmaterial about an axis normal to the direction in which the material isvibrating whereby the volume of the material is reduced.

5. The method of producing an article from powdered materials whichcomprises placing the powdered materials in a mold, subjecting thematerials to a tamping force and vibratory mo tion and simultaneouslywith the same rotating the powdered materials in a direction normalto-the direction of said motion to compact the materials and thereaftersintering the same to form an article of a coherent solid mass andhaving a shape corresponding to that of the mold. V

6. An apparatus for producing an article from powdered materialscomprising an arm mounted for rotation, a supporting means pivoted tosaid arm, a resilient member on said supporting means, a mold supportedby said resilient member and adapted to receive powdered material, aweighted closure member slidably mounted within said mold for closingthe same and for exerting a force upon the powdered material when thesaid arm is rotated and means to vibrate said mold and said weightedclosure member whereby said powdered material will be compacted.

7. The method of producing an article from powdered materials whichcomprises placing the powdered material in a mold, applying pressure tosaid material both by. centrifugal force and simultaneously by avibrating weight moving in a direction parallel to said force wherebythe material is compacted in the mold, and thereafter sintering thecompacted material'to form an article of a coherent solid mass.

8. An apparatus for producing an article from powdered materialscomprising an arm mounted for horizontal rotation, a supporting meanspivoted to an end of said arm, a resilient member carried by saidsupporting means, a mold supported by said resilient member and adaptedto receive powdered material, a weight slidably mounted in said mold formovement towards and away from said resilient member for reducing thevolume of the powdered material and means to vibrate said mold and saidweight whereby said powdered material will be compacted.

9. The method of producing a porous article by powder metallurgy whichcomprises placing in a mold a mass of powdered material of controlledpowder size throughout, applying pressure to said mass by centrifugalforce while subjecting said mass to vibratory motion in the direction ofsaid force whereby the mass is compacted in the mold, and thereaftersintering the' compacted mass to form a porous coherent solid bodyhaving a shape corresponding to that of the mold.

HORACE H. LESTER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Principles of Powder Metallurgyby W. D. Jones, published by Edward Arnold and Co., London, 1937, pages4'? and 48.

